PAN, also known as systemic necrotic vasculitis, was the first recognized form of primary systemic vasculitis. PAN causes the inflammation of small- to medium-sized blood vessels, especially those supplying the nerves, skin, kidneys, gastrointestinal tract, heart, eye, and genitals. Unlike another form of vasculitis called microscopic polyangiitis, PAN does not usually cause glomerulonephritis, a type of kidney disease, or vasculitis in the very smallest blood vessels (arterioles, capillaries, and venules). There are no radiographic or serologic tests that can reliably measure disease activity in PAN. Currently, clinicians must rely on patients' symptoms, signs, laboratory tests, and imaging to guide treatment decisions, but such data are rarely consistently reliable in determining PAN disease activity. This study will use new scientific methods to discover new biomarkers that can be used to monitor disease activity in PAN patients. These biomarkers may be used to help direct clinical care for PAN patients and assist in future drug development.